Active studies have been conducted continuously to develop novel refrigerants substituting for the existing refrigerants due to the destruction of the ozone layer and the problem of global warming.
Chlorofluorocarbon (CFC) compounds, which are not toxic to the human body and non-combustible, have high thermal and chemical stability, and thus are used widely in various industrial fields, including refrigerants, foaming agents, spraying agents, cleaning agents, or the like. CFC is also referred to as freon gas. CFC was discovered by Thomas Midgley (US) in 1928. However, since it was shown that CFC is broken into chlorine atoms by the solar UV rays to serve as a main cause of the destruction of the ozone layer, use of CFC has been restricted by the international regulation. Therefore, many studies which are conducted to develop novel refrigerants substituting for CFC have resulted in the finding of hydrofluorocarbon (HFC). HFC compounds include HFC-134a, HFC-152a, HFC-32, HJC-125, or the like. Among those, HFC-134a used widely as a refrigerant for cars has a low index of destruction of the ozone layer but is problematic in that it has a high global warming index, and thus EU prohibits the use of HFC-134a. It seems that the use of HFC-134a is gradually prohibited all over the world. Therefore, there has been a need for a novel refrigerant which does not adversely affect global warming as well as the destruction of the ozone layer. As an alternate refrigerant, 2,3,3,3-tetrafluoro-1-propene (HFO-1234yf), one of the HFC compounds, shows a low effect upon the destruction of the ozone layer and global warming, and thus is given many attentions as an eco-friendly refrigerant. HFO-1234yf is produced through a 4-step process including a hydrogenation of hexafluoropropylene (HFP) [K. Avril, B. Collier, U.S. Pat. No. 0,021,849 A1 (2011)]. Consequently, there is an imminent need for developing a high-quality catalyst for each step to accomplish cost-efficient production of HFO-1234yf.
Palladium is an active material used for various reaction steps and is used, for example, for a hydrogenation/dehydrogenation, removal of contaminants from car exhaust and cracking of petroleum [M. Fernandez-Garcia, A. Martinez-Arias, L. N. Salamanca, J. M. Coronado, J. A. Anderson, J. C. Conesa, and J. Soria, J. Catal., 187, 474 (1999); Y. Nishihata, J. Mizuki, T. Akao, H. Tanaka, M. Uenishi, M. Kimura, T. Okamoto, and N. Hamada, Nature, 418, 164 (2002); J. M. Thomas, B. F. G. Johnson, R. Raja, G. Sankar, and P. A. Midgley, Acc. Chem. Res., 36, 20 (2003)]. Particularly, palladium is used widely as a catalyst for organic synthesis reactions including a generation of a large amount of carbon-carbon bonds, such as Suzuki, Heck and Stille reaction [M. T. Reetz, and E. Westermann, Angew. Chem, Int. Ed., 39, 165 (2000); Y. Li, X. M. Hong, D. M. Collard, and M. A. EI-Sayed, Org. Lett., 2, 2385 (2000); S.-W. Kim, M. Kim, W. Y. Lee, and T. Hyeon, J. Am. Chem. Soc., 124, 7642 (2002)]. Herein, it is to be noted that the size and shape of a palladium catalyst become an important factor in determining the quality of catalyst for a reaction. For this, many studies have been conducted to provide palladium having various shapes. In most cases, many different organic solvents and surfactants are used. However, since such organic solvents are not eco-friendly, many attentions are given to an ionic liquid as a new eco-friendly solvent.
An ionic liquid is formed of ions only, and generally includes nitrogen-containing cations and relatively smaller anions. By virtue of such a structure, an ionic liquid has a low melting point, thermal stability, low volatility and high ion conductivity, and shows high solubility to metals, organic substances and organometallic compounds [P. Wasserscheid, and W. Keim, Angew. Chem. Int. Ed., 39, 3773 (2000); T. Welton, Chem. Rev., 99, 2071 (1999)].